is convex, plus some regularity conditions (see, for example [1]}) (\(\Vert .\Vert \)  is the Euclidean norm). Similarly, Azaïs and Wschebor [2]} use the condition:
\(\kappa (S)= \underset{t\in S}{\sup } \underset{s\in S,\; s\ne t}{\sup } \frac{\textnormal {dist}(s-t,C_t)}{\Vert s-t\Vert ^2}<\infty \) 
